This type of exhaust pipe structure is disclosed in PTL 1, PTL 2, or the like.
PTL 1 discloses an exhaust manifold of a four cylinder engine. According to this exhaust manifold, a mixing chamber having an ellipsoidal shape is arranged next to exhaust ports lined up in a row, and a catalyst converter is continuously provided immediately under the mixing chamber. Four exhaust pipes extending from the respective exhaust ports are connected to an outer periphery of the mixing chamber.
PTL 2 discloses an exhaust pipe structure which diffuses the flow of exhaust gas flowing into a catalyst converter. According to this exhaust pipe structure, a collecting pipe (rotational flow generator) is provided between a collecting portion, where the exhaust pipes extending from the exhaust ports are collected, and the catalyst converter. The collecting pipe adjusts the flow of the exhaust gas from the exhaust pipes to convert the flow into rotational flow.
In each of PTLs 1 and 2, the catalyst converter is arranged at a middle portion in the cylinder row direction.
A pipe structure configured such that the catalyst converter is slightly offset from the middle portion in the cylinder row direction is also disclosed (PTL 3). The exhaust pipes are connected to a periphery of the ellipsoid-shaped collecting portion which is continuous with the catalyst converter, and the exhaust pipes are designed such that the exhaust gas flowing through each exhaust pipe flows toward the center of the collecting portion.